4chan users have speculated that the mysterious post was some kind of coded warning, threat – or instruction.

4CHAN

MYSTERY: The 4chan post (at the top) is just a picture of guns, dots and dashes

The owner of 4chan said the website was “ready to respond” to police.

The unknown user posted a series dots and dashes with an image of two handguns.

The message was Morse code, and when deciphered, it said: “Go to http://pastebin.com/zluxdmza”.

PASTEBIN

LINK: A Pastebin post that contains the GPS coordinates of a terror attack

When readers followed the link, they found a binary code of 0s and 1s.

When that was deciphered it was a GPS location near Big Ben.

The originally 4chan message was posted at 3pm on Tuesday – almost exactly 24 hours before the car and knife attack on Westminster Bridge and Parliament unfolded.

Terror in the HEART of London: the Westminster attack as it happened

The attack in Westminster has left four people dead and up to 20 injured after a man drove his vehicle into pedestrians, then ran into the grounds of the Houses of Parliament before being shot by police

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PA

The alleged attacker, with the suspected weapons used in the attack highlighted in red

4chan users cracked the code ahead of the attack and couldn’t believe their eyes when the horror unfolded at the exact location identified.

The poster’s location was given as Denmark – but he or she could have used a simple web technique to disguise their true location.

4chan owner Hiroyuki Nishimura told Gizmodo, which first revealed the post: "I understand the situation. We are ready to respond for law enforcement agency."